Fuel/electric hybrid powertrain architectures comprise torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to a vehicle driveline. One such transmission includes a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving motive torque from an internal combustion engine, and an output member for delivering motive torque from the transmission to the vehicle driveline. Exemplary electro-mechanical transmissions are selectively operative in fixed gear modes and continuously variable modes through actuation of torque-transfer clutches. A fixed gear mode occurs when rotational speed of the transmission output member is a fixed ratio of rotational speed of the input member from the engine, typically due to actuation of one or more torque-transfer clutches. A continuously variable mode occurs when rotational speed of the transmission output member is variable based upon operating speeds of one or more electrical machines. The electrical machines can be connected to the output shaft via actuation of a clutch, or by direct connection. Clutch actuation and deactivation is typically effected through a hydraulic circuit, including electrically-actuated hydraulic flow management valves, pressure control solenoids, and pressure monitoring devices controlled by a control module.
Engineers implementing powertrain systems having electro-mechanical transmissions are tasked with developing shifting schemes between various operating modes, including the fixed gear modes and the continuously variable modes. Execution of a shift typically includes deactivating an off-going clutch, and continuing operation or actuating an oncoming clutch. A control system is typically programmed to execute tasks during a transmission shift to effect a smooth shift transition substantially imperceptible to the vehicle operator. Such control tasks include synchronizing clutch speed of the oncoming clutch by controlling operating parameters of the engine and electrical machines. Operating parameters include torque outputs and operating speeds of the engine and electrical machines. However, if a fault were to unknowingly occur in deactivating the off-going clutch, the control system may act to control the operating parameters of the engine and electrical machines to effect the shift transition even though the off-going clutch has not deactivated, potentially resulting in operator dissatisfaction.
Therefore, there is a need to for a method and apparatus to control operation of a hybrid transmission during gear shifting events, to address concerns mentioned hereinabove.